In the manufacture of roll formed steel articles the feedstock may be hot rolled or cold rolled steel strip. Hot rolled steel strip when supplied from the steel mill is coated with a tough skin of steel oxide (predominately Fe.sub.3 O.sub.4) known as "mill scale" oxide while cold rolled steel is usually supplied with a smooth surface free of blemishes ("bright" steel).
To enable the later application of decorative or protective surface finishes such as paint or electroplating to the roll formed article it is customary to remove mill scale from hot rolled strip by immersion in a pickling bath. Pickling may be carried out on the coil of steel strip before the roll forming process or the roll formed articles may be pickled prior to surface finishing.
The pickling process however has largely fallen into disfavour due to its excessive and costly consumption of time, energy and pickling chemicals and at the same time creates an environmental problem in the disposal of spent chemicals. Further difficulties associated with the pickling process relate to the creation of surface defects such as pitting in the treated surface and the effective de-passivation of the steel leading the surface corrosion on the strip coils before rolling as well as on the roll formed products. Although surface corrosion may be alleviated by treating the strip coils or the roll formed articles with protective film of a mineral or synthetic oil, there are further costs and environmental problems associated with the removal and disposal of the protective oil.
In order to alleviate the problems associated with roll forming of hot rolled steel, it has been proposed to produce a clean bright steel by cold rolling the hot rolled strip.
In cold rolling of steel strip, the coil may be pickled to remove the major portion of adhering mill scale and then passed through a four or five roll mill to produce a steel strip of superior dimensional tolerances. During the cold rolling operation a lubricant such as mineral oil or synthetic oil is used and this must be removed from the surface of the strip before or during the subsequent high temperature annealing process to avoid surface carburization.
Lubricant removal may be effected by a variety of methods. The coils, coated in lubricant, may be annealed in a carefully controlled atmosphere of steam, nitrogen and hydrogen to remove the carbon residues from the surface of the strip. Alternatively the rolling mill lubricant may comprise a low boiling point composition which evaporates during the annealing process.
Other lubricant removal processes are effected by spraying onto the surface of the steel strip detergent compositions either just before entry into the last roll stand or just after exit. Although detergents are generally satisfactory for removal of lubricant and adhered metal particles from the strip surface, the use of detergents is known to cause problems in a cold rolling mill due to a marked decrease in lubricity which in turn causes production of a large amount of metallic particles on the strip surface.
The problems associated with the use of detergents for removal of lubricating oils are addressed in Austrailian Pat. No. 552870. In Australian Pat. No. 552870 it is stated that the "problems caused by the detergent method" using water or a detergent can be solved by injecting at high pressure a rolling mill lubricant emuslion of the concentration of 0.5-2.0% onto the surface of the strip at the entrance or exit of the last stand and injecting at a low pressure a rolling mill lubricant emulsion of the same concentration to the rolls as a lubricant.
Australian Pat. No. 552870 also suggests that in the prior art high pressure detergent method, an oil separator is necessary for recovery of the oil and detergent for later use.
Although certain economies may be achieved by utilizing hot rolled steel strip as feedstock for roll forming, most of these economies are lost with prior art processes. Roll formed articles produced from hot rolled steel strip posses a rough surface due to the adhesion of mill scale flakes as well as a certain amount of residual mill lubricant. Such roll formed articles often exhibit a considerable degree of surface rust after subsequent storage and transportation.
Accordingly before such roll formed articles may be utilized in structural applications or other applications including furniture manufacture, considerable time, effort and cost is expended in preparing the surface of the articles to accept a protective or decorative surface finish such as paint or the like. Residual mill scale must be removed by pickling or brushing, sand blasting or the like and residual lubricant may be removed by solvent or alkali stripping processes.
In the light of the high costs associated with the subsequent surface finishing of articles roll formed from hot rolled steel strip most roll forming processors prefer to use cold rolled steel strip notwithstanding its higher initial cost as a feedstock. Cold rolled steel strip is usually supplied to a roll forming mill with a protective coating of mineral oil or the like to prevent surface rusting during transportation and storage prior to the roll forming operation.
Similarly, the roll formed product is usually supplied to a steel fabricator with a protective coating of mineral oil to prevent surface rusting. The steel fabricator must therefore remove the oil film by expensive solvent or alkali stripping processes and then the roll formed product has a protective primer applied thereto by hand brushing, spray painting or the like.
Although it has previously been proposed to apply a surface coating such as a metal primer paint to roll formed articles during the roll forming process, such prior art "in line" rolling and painting processes have been quite unsatisfatory.
In prior art processes using hot rolled steel strip it has been proposed to use and sand blasting and alkali treatment stations after the rolling mill and then to paint the severed lengths "off line". This process is extremely inefficient as in order for the sand blasting and alkali treatment processes to be effective the line speed of the rolling mill must be substantially reduced. In addition the process consumes large amounts of energy and materials and creates environmental problems in disposal of spent chemicals. A further difficulty arises in the need for additional storage and handling space to paint the sections off line with consequent high additional labour costs and environmental problems caused by evaporating solvents in the paint region.